Basic fault finding for advanced emission test failures:
- If a vehicle fails for high CO at idle and/or fast idle then... if Lambda is too low, the mixture is too rich. This can be caused either by a misfire or by a faulty Lambda sensor.
- If it's running very rich, the 02 and HC will be higher than normal.
- If it has a high Lambda and high 02 (0.5 to 1.5%), the "CAT" could be faulty. Before condemning it make sure its hot enough to work, the Lambda and other sensors are working OK and the exhaust downpipe hasn't got any leaks.
- If the vehicle fails for high HC then... if the CO passes, the vehicle is probably misfiring. Or if the CO fails and the Lambda reading is too low, the mixture is probably too rich and it may also be misfiring.
- If the CO fails, the Lambda reading is high and the O2 reading is high (0.5 to 1.5%) the "CAT" could be faulty. Again check out the simple measures above before condemning it.
- If the Lambda fails then... if its too high the Lambda sensor may be faulty, the mixture may be too weak or there may be a hole in the exhaust downpipe. If Lambda is too low the sensor may be faulty, the mixture may be rich or the vehicle may be misfiring.
I spoke with the guy who tested the car and asked him for some more details. He confirmed that it was the high idle test (2500 to 3200 rpm) test that failed, with high CO. He said that he also measured the mixture and that this was OK... apparently the mixture needs to be between 0.95 and 1.05 (something like that) on the Lambda to pass the test.
Looks like I need to measure O2 and HC to really get to the bottom of this one, after confirming that the mixture is OK on Monday. Given that I've had some problems with the Lamda sensor connection to the exhaust, the problem may lie with the Lamda sensor (item 32 in the drawing below). You can also get some good background on Lambda sensors on the Pico website.
If I have to replace the Lambda sensor then I'm tempted to install a wideband device with some data logging functionality, since this would be very useful for keeping an eye on the engine and making sure that it's running properly, rather than waiting for things to go wrong. It would also enable me to make upgrades more safely. I'd probably consider something like the PLX Devices R-500.
I hope it's not the catalysts!